1. Field of the Invention
The present invention relates to a light-emitting element package device, and particularly to a light-emitting element package device with a reduced total reflection and Fresnel loss.
2. Description of the Prior Art
Recently, the new application fields of light-emitting diodes (LEDs) have been developed. Different from a common incandescent light, a cold illumination LED has the advantages of low power consumption, long device lifetime, no idling time, and quick response speed. In addition, the LED also has the advantages of small size, shock resistance, being suitable for mass production, and being easily fabricated as a tiny device or an array device. A common LED package typically comprises an LED element for illumination. When light is emitted from the LED, a series of procedures including diffusing, reflecting, mixing, or light wavelength conversion proceed in a molding material or encapsulation material to generate satisfactory hues and brightness. Therefore, the selection of the molding material or encapsulating material is important to design an LED package.
As shown in FIG. 1, a conventional surface mount type (SMD) LED package device 10 is typically packaged with a transparent encapsulation material 12 to protect an LED chip 14, conducting wires (not shown), and the like disposed on a lead frame 13. The material used for the encapsulation is usually uniform and has a single refraction index. According to the theory of Fresnel loss, the light extraction efficiency equals to 4/[2+(n1/n2)+(n2/n1)], wherein n1 and n2 represent the refraction indexes of two materials bordered and having an interface therebetween. When an interface formed between two different materials, the more the difference between the two refraction indexes is, the more the Fresnel loss is. As shown in FIG. 1, when a light is emitted from the LED chip 14 (having, for example, a refraction index of 2 to 3.4) and incident onto the encapsulation material 12 through the first interface 16, or further traveled from the encapsulation material 12 (having, for example, a refraction index of 1.3 to 1.5) and incident onto the air (having, for example, a refraction index of 1) of the environmental ambiance through the second interface 18. Fresnel loss occurs at the two interfaces. Furthermore, at the interface, a total reflection loss may also occur when a light is incident onto a material having a low refraction index (n2) from a material having a high refraction index (n1). The light extraction efficiency is proportional to (n2/n1)2. The term “light extraction efficiency” means the number of photons actually measured at the exterior of the light-emitting device, which are produced by the LED and remains after the absorption, refraction, and reflection by the light-emitting device per se. Generally, when two materials have an interface therebetween, the more the difference of the refraction indexes is, the more the loss of light is.
Another conventional surface mount type light-emitting diode package device 20, as shown in FIG. 2, is a multi-chip package and has a plurality of LED chips 14 emitting lights with various colors. A diffuser 22, such as, aluminum oxide and the like, is usually used and uniformly distributed in the encapsulation material 12 in order to increase color mixing. However, the light intensity, on the contrary, is decreased due to shielding effect.
Accordingly, U.S. Patent Application No. 2006/0220046A1 discloses an LED light-mixing package as shown in FIG. 3. The LED light-mixing package 30 is a surface mount type LED package comprising a package seat 32 having a holding space for holding LED chips. At least a red LED chip 34, at least a blue LED chip 35, and at least a green LED chip 36 are disposed on the package seat 32, wherein each chip is electrically connected to an electrode 33 of the package seat 32 through a wire (not shown) and may be electrically connected to an external element through the package seat 32. The LED light-mixing package 30 further comprises a sealing member 37 covering the chips and filling the holding space of the package seat 32. The LED light-mixing package 30 also comprises a plurality of diffuser particles 38 distributed in the sealing member 37. The materials of the diffuser particles 38 have high reflectivity or high light-scattering properties and are selected from the group consisting of silver, resin, silicon, and other white inorganic compounds for scattering and mixing lights. Therefore, red light, blue light, and green light emitted from the chips can be mixed to produce white light through spreading in sealing member 37 after hitting the diffuser particles 38. The diffuser particles 38 located on different positions in the sealing member 37 may have different distribution densities and amounts for providing preferable scattering routs of the three kinds of color light so as to efficiently mix light. The distribution density of the diffuser particles 38 far from the chips is greater than the distribution density of the diffuser particles 38 close to the chips. In other words, the amount of the diffuser particles 38 far from the chips is more than that close to the chips. Such diffuser particles in the LED light-mixing package are used for light mixing, and the distribution density is intentionally reduced above the chips to decrease the loss of light intensity caused by shielding effect.
However, Fresnel loss and total reflection loss still compose a large portion of the light loss of LED packages, such that an improvement of the light extraction efficiency is still desired. Therefore, there is still a need for a light-emitting element package having excellent light extraction efficiency.